The invention relates to pressure sensitive adhesives based on block copolymers, said block copolymers comprising at least the unit P(A)-P(B)-P(A), composed of a middle polymer block P(B) and of two polymer blocks P(A) surrounding the middle polymer block P(B), or the unit P(B)-P(A)-P(B), composed of a middle polymer block P(A) and of two polymer blocks P(B) surrounding the middle polymer block P(A), to the use of such adhesives, and to a process for preparing them.
In industry, hotmelt processes operating with solventless coating technology are of growing importance in the preparation of pressure sensitive adhesives. In general, environmental regulations and increasing costs are forcing forward the development process of such adhesives. Besides SIS (styrene-isoprene-styrene copolymers) systems, acrylic polymers are increasingly being applied from the melt as a polymer film to backing materials. Moreover, for specialty applications, pressure sensitive adhesive tapes which feature very low outgassing are needed. This is something which can be ensured only by means of hotmelt processes, since conventional coatings applied from solution always still contain small fractions of residual solvent.
Additionally, there is a growing need for acrylic pressure sensitive adhesives on apolar surfaces. In industry, the use of plastics is on the increase, owing to the weight reduction they offer as compared with conventional metals, and, accordingly, adhesive bonds are being formed more and more on these substrates. Since the large number of their ester groups makes polyacrylates relatively polar, bond strength can be increased only by using resins of similar polarity. Although these resins exhibit good bond strength on polar surfaces such as steel, their bond strengths to apolar surfaces are mediocre and inadequate for the majority of applications.
U.S. Pat. No. 4,418,120 A describes crosslinked pressure sensitive adhesives comprising rosin ester resins, which possess good bond strength to polypropylene. Owing to the unsaturated nature of the resin, however, the UV stability of these adhesives is poor. The bond strengths to the apolar surfaces as well are relatively low.
U.S. Pat. No. 4,726,982 A describes crosslinked pressure sensitive adhesives featuring high bond strength to inks and paints. The copolymers of acrylic esters and N-vinyl-2-pyrrolidone are blended with tackifiers such as poly(isobornyl methacrylate), pentaerythritol esters of rosins, and mixed aliphatic/aromatic resins.
Here again, relatively polar resins are used to increase the bond strength to apolar surfaces. The tackifiers mostly described, however, possess unsaturated compounds which, in the hotmelt process, can lead to, instances of gelling; additionally, after bonding, the unsaturated compounds undergo aging or weathering under UV light and, accordingly, the adhesive properties suffer over a prolonged period of time.
In EP 0 707 604 A1, polyethylene/butylene macromonomers are used for copolymerization with acrylates. As a result, phases are formed which have a low glass transition temperature, which in turn allow the adhesives to flow on apolar surfaces and thus ensure high bond strengths to PE and PP. A disadvantage is the poor conversion of the polymerization process described. Furthermore, the macromonomers are copolymerized randomly, and so no domains are able to form. Such domains would offer regions in which even very apolar resins would be soluble. With these pressure sensitive adhesive tapes, therefore, only a relatively low bond strength to apolar surfaces is attained.
Furthermore, the polyacrylates described are very difficult to process as hotmelts because the high residual monomer fractions impact negatively on the concentration process, and migration within the pressure sensitive adhesive tape can have an adverse effect on the long-term adhesive properties.
In contrast, styrene-isoprene-styrene (SIS) block copolymers are widespread as elastomers for hotmelt-processable pressure sensitive adhesives [preparation processes:
U.S. Pat. No. 3,468,972 A; U.S. Pat. Nos. 3,595,941 A; use in pressure sensitive adhesives: 3,239,478 A; U.S. Pat. No. 3,935,338 A] with high bond strength to apolar surfaces. Their effective processability is achieved by means of a relatively low molecular weight and by means of a specific morphology [EP 0 451 920 B1]. These pressure sensitive adhesives can be crosslinked very effectively with UV light in the presence of photoinitiators or with electron beams, since the middle blocks contain a large number of double bonds.
Nevertheless, these elastomers are not without their disadvantages, such as, for example, severe aging under UV light (i.e., in daylight too) and in an oxygen/ozone atmosphere. Moreover, owing to the formation of hard block domains comprising the hard polystyrene end blocks, effective flow on apolar surfaces is prevented. The same applies to other block copolymers which possess a middle block containing at least one double bond [U.S. Pat. No. 5,851,664 A].
Another very unfavorable property of styrene-isoprene-styrene block copolymers for the application is the relatively low thermal shear strength. These pressure sensitive adhesives, accordingly, are unsuited to applications within relatively high temperature ranges.
U.S. Pat. No. 5,314,962 A describes A-B-A block copolymers as elastomers for adhesives, which possess A domain formation as their cohesion-forming feature. As a result of the selection of the comonomers used, however, only low bond strengths can be realized on apolar surfaces. Additionally, these polymers are not compatible with highly apolar resins.
EP 0 921 170 A1 describes A-B-A block copolymers which have been modified with additions of resin. Here again, owing to the selection of the comonomers and added resins, only low bond strengths can be realized on apolar surfaces.
EP 0 408 429 A1 and EP 0 408 420 A1 described A-B-A block copolymers which, however, were synthesized by living anionic polymerization. Because of the absence of an acrylic acid fraction, however, these polymers are unsuited to use as pressure sensitive adhesives, since the internal cohesion of the middle block is too low and at least a low fraction of polar monomers is required for bonds to steel. Because of the anionic polymerization, it is not possible to use acrylic acid comonomers or other hydroxyl-functionalized acrylate comonomers, such as hydroxyethyl acrylate, for example. And, again, highly efficient flow on apolar surfaces is prevented by the use of the conventional domain-forming polymers, such as PMMA or polystyrene.
In U.S. Pat. No. 5,166,274 A, this acrylic acid deficiency is compensated by hydrolyzing block copolymers of tert-butyl methacrylate in order to liberate the carboxylic acid function. On the industrial scale, however, this method cannot be applied to pressure sensitive adhesives, since the hydrolysis step is very costly and time consuming.
In U.S. Pat. No. 6,069,205 A1, diblock and triblock copolymers are described which are prepared by an atom transfer polymerization and utilized for adhesives. This method too is unsuitable for preparing pressure sensitive adhesives, since it uses relatively high catalytic amounts of heavy metal compounds which would have to be removed, in a cumbersome operation, by extraction processes.
EP 1 008 640 A1 describes styrene block copolymers comprising an acrylate middle block composed, however, of the common C2 to C14 alkyl acrylates. Because of the restriction of the comonomers and, associated therewith, the restriction of the resins which can be used, only low bond strengths to apolar substrates are achievable with these polymers. Moreover, metal salts are used to prepare these polymers too (in analogy to U.S. Pat. No. 6,069,205 A), which would have to be removed, again a cumbersome operation, for pressure sensitive adhesive tape applications.
It is an object of the invention to provide improved pressure sensitive adhesives based on polyacrylate which do not have the disadvantages of the prior art, or in which said disadvantages are reduced.
Surprisingly and unforeseeably for the skilled worker, this object is achieved by the inventive pressure-sensitive adhesives as specified in the claims. The main claims relate in particular to pressure sensitive adhesives which have an aging behavior better than that of SIS compositions, a bond strength to apolar surfaces which is higher than that of the conventional, prior art A-B-A polyacrylate pressure sensitive adhesives, and an excellent compatibility with very apolar resins.
The invention accordingly provides a pressure sensitive adhesive based on block copolymers, said block copolymers having at least one unit composed of three successive polymer blocks, said three successive polymer blocks being chosen in alternation from the group of the polymer blocks P(A) and P(B), wherein
P(A) represents a homopolymer or copolymer block obtainable from a component A which is composed of at least one monomer A1,
said at least one monomer A1 being an acrylated macromonomer of the general formula
CH2xe2x95x90CH(RI)(COORII)xe2x80x83xe2x80x83(I)
in which RI=H or CH3 and RII is an aliphatic linear, branched or cyclic, unsubstituted or substituted, saturated or unsaturated alkyl radical having more than 30 atoms, the average molecular weight Mn of said at least one macromonomer being between 492 g/mol and 30 000 g/mol,
P(B) represents a homopolymer or copolymer block obtainable from a component B which is composed of at least one monomer B1, the polymer block P(B) having a softening temperature of from xe2x88x9280xc2x0 C. to +20xc2x0 C., and
the polymer blocks P(A) are not homogeneously miscible with the polymer blocks P(B).
Accordingly, the block copolymers of the inventive pressure sensitive adhesive comprise at least the unit P(A)-P(B)-P(A) composed of a middle polymer block P(B) and of two polymer blocks P(A) enclosing the middle polymer block P(B), and/or the structural unit which is inverse to this, i.e., at least the unit P(B)-P(A)-P(B) composed of a middle polymer block P(A) and of two polymer blocks P(B) enclosing the middle polymer block P(A), subject to the above-described provisos for the polymer blocks P(A) and P(B).
The block copolymers for the pressure sensitive adhesive of the invention can be prepared and used to advantage in a very wide variety of structures. Explicitly, reference may be made to linear or branched chains of the polymer blocks P(A) and P(B), corresponding for example to a general structure [P(A)-P(B)]n, to star polymers of P(A) and P(B), corresponding for example to the general structures [[P(A)-P(B)]n]mX or [[P(B)-P(A)]n]mX or [[P(A)-P(B)]n]mX[P(A)-P(B)]p]q, or to the general structures [[P(A)-P(B)]n]mXLp or [[P(B)-P(A)]n]mXLp, in which L here can denote different radicals. The enumeration of these structures is intended only by way of example, without representing any limitation. The structures also embrace all of the xe2x80x9casymmetricxe2x80x9d structures in which all of the polymer blocks P(A) and P(B) meet per se the definitions set out above, but in which the indices n, m, p and q imply only the multiple occurrence of the individual units but not their chemical or structural identity. L can with preference be radicals which, for example, represent poly(meth)acrylates but which do not inherently have a block structure. Also included here are homopolymers of the components A or B.
In one procedure which is advantageous in the sense of the invention, macromonoers A1 used comprise hydrogenated ethylene/propylene macromonomers and/or hydrogenated ethylene/butylene macromonomers which possess a glass transition temperature of not more than 0xc2x0 C. and carry an acrylate or methacrylate end group.
One very preferred example used is Kraton L-1253(trademark) (Shell AG). Kraton Liquid L-1253(trademark) (Shell AG) is a macromonomer having a molecular weight of 4 000 g/mol, is methacrylate functionalized, and possesses a saturated poly(ethylene/butylene) unit as aliphatic side chain.
In a procedure which is very advantageous in the sense of the invention, the macromonomers used comprise aliphatic acrylates, such as triacontanyl acrylate, and also higher homologs.
In order to obtain good immiscibility of the polymer blocks P(A) and of the polymer blocks P(B) it is of advantage to make these blocks highly different in polarity and so to ensure optimum phase formation. It is therefore advantageous to choose preferably apolar monomers as a further monomers of component A for the synthesis of the polymer blocks P(A).
For improved phase separation it is also possible to copolymerize monomers having a high glass transition temperature for the polymer blocks P(A). Advantageous examples which are used as comonomers for component A1 are vinylaromatics, methyl methacrylates, cyclohexyl methacrylates, and isobornyl methacrylates. Particularly preferred examples are methyl methacrylate and styrene.
As component A it is also possible to use mixtures of the above-described macromonomers with the monomers set out below that are used to form the polymer blocks P(B), provided the mixing proportions are chosen so that the phase separation of the polymer blocks P(A) and P(B) is still ensured.
In an advantageous development of the pressure sensitive adhesive of the invention, component B is composed at least in part of monomers B1 which are chosen from the following groups:
(a) acrylic and methacrylic acid derivatives of the general formula
CH2xe2x95x90CH(RIII)(COORIV)xe2x80x83xe2x80x83(II)
in which RIII=H or CH3 and RIV represents a linear or branched aliphatic alkyl chain having from 2 to 20, preferably from 4 to 14, in particular from 4 to 9 carbon atoms,
(b) vinyl compounds, especially those which have one or more functional groups capable of crosslinking,
component B being composed in particular of from 60 to 100% by weight of compounds from group (a) and from 0 to 40% by weight of compounds from group (b).
In one advantageous variant of the pressure sensitive adhesive of the invention, it is possible for group (a) to use acrylic monomers of the general formula (II) in which the group xe2x80x94ORIV constitutes or comprises a functional group for crosslinking of the pressure sensitive adhesive.
Preferred examples of group (a) monomers are n-butyl acrylate, n-pentyl acrylate, n-hexyl acrylate, n-heptyl acrylate, n-octyl acrylate, n-nonyl acrylate and their branched isomers, such as 2-ethylhexyl acrylate, for example. Furthermore, the methacrylates corresponding to the abovementioned acrylates are among preferred examples of the group (a) monomers. Compounds which lend themselves outstandingly to use as group (a) monomers are, additionally, isobutyl acrylate, isooctyl acrylate, and isobornyl acrylate.
Vinyl compound group (b) monomers here are all monomers containing a vinylogous double bond capable of polymerization, especially those in which this double bond is activated for polymerization by functional groups. In this sense it is also possible to classify (meth)acrylates within the group of the vinyl monomers. In respect of group (b) it is preferred to use monomers which lower the glass transition temperature of the polymer block P(B), alone or in combination with other monomers, especially those from groups (a) or (b), to below 20xc2x0 C.
In one very advantageous embodiment of the invention, at least one of the monomers of component B, especially at least one of the group (b) monomers, is chosen such that it contains one or more functional groups which can be used for a crosslinking reaction of the block copolymer, especially for a thermal or radiation-chemical crosslinking, and, very particularly, for a crosslinking which is induced and/or assisted by UV radiation or by irradiation with electron beams.
With particular advantage, these can be (meth)acrylic derivatives containing unsaturated alkyl radicals in the radical RVI, of the general formula
CH2xe2x95x90CH(RV)(COORVI)xe2x80x83xe2x80x83(III)
in which RV=H or CH3. Preferred for RVI are alkyl radicals having from 3 to 14 carbon atoms which contain at least one C-C double bond. For acrylates modified with double bonds, allyl acrylate and acrylated cinnamates are particularly advantageous.
Additionally here and with great preference it is also possible to use acrylic monomers of the general formula (III) in which the group xe2x80x94ORVI represents another functional group for crosslinking of the adhesive or comprises one or more further and/or different functional groups for crosslinking of the adhesive.
Moreover, it is also possible, very advantageously, to use as group (b) monomers vinyl compounds having further double bonds which do not react during the (radical) polymerization. Particularly preferred examples are isoprene and butadiene.
Preferred examples of group (b) monomers further include vinyl acetate, acrylamides, photoinitiaters functionalized with at least one double bond, tetrahydrofuryl acrylate, hydroxy-functionalized (meth)acrylates, carboxyl-functionalized (meth)acrylates, amine- or amide-functionalized (meth)acrylates, and also vinyl esters, vinyl ethers, vinyl halides, vinylidene halides, vinyl compounds containing aromatic ring systems and heterocyclic systems in xcex1 position, especially vinyl acetate, vinylformamide, vinylpyridine, ethyl vinyl ether, vinyl chloride, vinylidene chloride, and acrylonitrile.
As group (b) monomers it is also possible to use, with advantage, hydroxyethyl acrylate, hydroxypropyl acrylate, hydroxyethyl methacrylate, hydroxypropyl ethacrylate, acrylic acid, methacrylic acid, allyl alcohol, maleic anhydride, itaconic anhydride, itaconic acid, benzoin acrylate, acrylated benzophenone, acrylamide, and glyceridyl methacrylate.
In another very advantageous embodiment of the inventive pressure sensitive adhesive, functional groups capable of crosslinking are introduced which are capable of a crosslinking reaction under the effect of thermal energy. Greatly preferred in this context are hydroxyl, carboxyl, epoxy, amide, acid isocyanato or amino groups.
For preparing the block copolymers used for the pressure sensitive adhesives of the invention it is possible in principle to use any polymerization which proceeds in accordance with a controlled-growth radical mechanism, such as, for example, GTRP (group-transfer radical polymerization), ATRP (atom-transfer radical polymerization), nitroxide/TEMPO controlled polymerization or, very preferably, the RAFT process (raid addition fragmentation chain transfer) or a modified RAFT process.
Alternatively, the polymers may also be prepared by a living anionic polymerization, sequentially or using a difunctional initiator. A prerequisite for this is that the monomers used do not include any compounds which might interrupt or terminate the anionic polymerization.
The polymerization can be conducted in the presence of an organic solvent or in the presence of water, or in mixtures of organic solvents and/or water, or else without solvent. It is preferred to use as little solvent as possible. Depending on conversion, temperature, and method, the polymerization time is between 6 and 72 hours.
In the case of solution polymerization, the solvents used are preferably esters of saturated carboxylic acids (such as ethyl acetate), aliphatic hydrocarbons (such as n- hexane or n-heptane), ketones (such as acetone or methyl ethyl ketone), aromatic solvents (such as toluene or xylene), special boiling point spirit, or mixtures of these solvents. For polymerization of the very apolar macromonomers it is very preferable to use for polymer block P(A) apolar solvents, such as aliphatic hydrocarbons or special boiling point spirits. For polymerization in aqueous media or in mixtures of organic and aqueous solvents, it is preferred to add emulsifiers and stabilizers for the polymerization. Polymerization initiators used are customary radical-forming compounds such as, for example, peroxides, azo compounds, and peroxosulfates. Initiator mixtures are also outstandingly suitable.
If the polymers are prepared by TEMPO- or nitroxide-controlled controlled-growth radical polymerization, then for the purpose of radical stabilization, nitroxides of type (IVa) or (IVb) are used: 
in which R1, R2, R3, R4, R5, R6, R7, and R8 independently of one another denote the following compounds or atoms:
i) halides, such as chlorine, bromine or iodine
ii) linear, branched, cyclic, and heterocyclic hydrocarbons having from 1 to 20 carbon atoms, which can be saturated, unsaturated, and aromatic,
iii) esters xe2x80x94COOR9, alkoxides xe2x80x94OR10 and/or phosphonates xe2x80x94PO(OR11)2, in which R9, R10 and R11 are radicals from group ii).
The compounds (IVa) or (IVb) may also be attached to polymer chains of any kind and can therefore be used for constructing the block copolymers, as macroradicals or macroregulators.
As controlled regulators for the polymerization it is more preferred to use compounds of the following types:
2,2,5,5-tetramethyl-1-pyrrolidinyloxyl (PROXYL), 3-carbamoyl-PROXYL, 2,2-dimethyl-4,5-cyclohexyl-PROXYL, 3-oxo-PROXYL, 3-hydroxylimine-PROXYL, 3-aminomethyl-PROXYL, 3-methoxy-PROXYL, 3-t-butyl-PROXYL, 3,4-di-t-butyl-PROXYL
2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO), 4-benzoyloxy-TEMPO, 4-methoxy-TEMPO, 4-chloro-TEMPO, 4-hydroxy-TEMPO, 4-oxo-TEMPO, 4-amino-TEMPO, 2,2,6,6,-tetraethyl-1-piperidinyloxyl, 2,2,6-trimethyl-6-ethyl-1-piperidinyloxyl
N-tert-butyl 1-phenyl-2-methylpropyl nitroxide
N-tert-butyl 1-(2-naphthyl)-2-methylpropyl nitroxide
N-tert-butyl 1-diethylphosphono-2,2-dimethylpropyl nitroxide
N-tert-butyl-1-dibenzylphosphono-2,2-dimethylpropyl nitroxide
N-(1-phenyl-2-methylpropyl) 1-diethylphosphono-1-methylethyl nitroxide
di-t-butyl nitroxide
diphenyl nitroxide
t-butyl t-amyl nitroxide.
As a further controlled polymerization method, use may be made of atom transfer radical polymerization (ATRP), in which preferably monofunctional or difunctional secondary or tertiary halides are used as initiators and the halide(s) is (are) abstracted using complexes of Cu, Ni, Fe, Pd, Pt, Ru, Os, Rh, Co, Ir, Cu, Ag or Au [EP 0 824 111 A1; EP 0 826 698 A1; EP 0 824 110 A1; EP 0 841 346 A1; EP 0 850 957 A1]. The various possibilities of ATRP are described further in the documents U.S. Pat. Nos. 5,945,491 A, 5,854,364 A, and 5,789,487 A.
The block copolymer can also be prepared by anionic polymerization. In this case the reaction medium used preferably comprises inert solvents, such as aliphatic and cycloaliphatic hydrocarbons, or else aromatic hydrocarbons, for example.
The living polymer is generally represented by the structure P(B)-M, in which M is a metal from Group I of the Periodic Table, such as lithium, sodium or potassium, for example. The molecular weight of the polymer is determined by the ratio of initiator to monomer. In order to construct the block structure, the monomers for block P(A) are added, after which the monomer(s) for block P(B) is (are) added in order to prepare the polymer block P(B)-P(A)-P(B). Alternatively, P(B)-P(A)-M can be coupled by a suitable difunctional compound. In this way, star block copolymers [P(B)-P(A)]nX are also obtainable. Examples of suitable polymerization initiators include n-propyllithium, n-butyllithium, 2-naphthyllithium, cyclohexyllithium and octyllithium, without wishing this enumeration to constitute any unnecessary restriction.
Furthermore, it is also possible to use difunctional initiators, such as 1,1,4,4-tetraphenyl-1,4-dilithiobutane or 1,1,4,4-tetraphenyl-1,4-dilithioisobutane. Coinitiators can also be used. Examples of suitable coinitiators include lithium halides, alkali metal alkoxides, and alkylaluminum compounds. This technique is also known as ligated anionic polymerization.
As a very preferred variant for the preparation a modified RAFT process (reversible addition-fragmentation chain transfer) is carried out. The RAFT process is described in detail in the documents WO 98/01478 A1 and WO 99/31144 A1, although the reaction regime described could not be used to prepare any materials suitable for pressure sensitive adhesives. Suitable with particular advantage for preparing the block copolymers used for the pressure sensitive adhesives of the invention are trithiocarbonates [Macromolecules 2000, 33, 243-245], in which case, in a first step, monomers for the end blocks are polymerized and, in a second step, the middle block is polymerized. Following the polymerization of the end blocks, the reaction can be terminated and reinitiated. As a result of repeated initiation the conversion achievedxe2x80x94in contrast to the original RAFT processxe2x80x94is good, so that the block copolymers prepared can also be used as acrylic pressure sensitive adhesives. It is also possible to carry out polymerization sequentially without interrupting the reaction. In one very advantageous variant, the trithiocarbonate (V) is used for the polymerization, particularly of acrylates: 
More suitable for the polymerization of methacrylates are trithiocarbonates of types (VI) and (VII) 
in which R12 and R13 can be identical or different and comprise H, methyl, halides such as chlorine, bromine or iodine, for example, linear, branched, cyclic, and heterocyclic hydrocarbons having 2 to 20 carbon atoms, which can be saturated, unsaturated or aromatic, esters xe2x80x94COOR, alkoxides xe2x80x94OR and heterocycles having up to 20 carbon atoms.
The block copolymer used for the pressure sensitive adhesives of the invention is processed further conventionally from solution or from the melt. For processing from the melt, the block copolymer is removed from the solvent in a concentrative extruder under reduced pressure, in which case it is possible, for example, to use single-screw or twin-screw extruders. Twin-screw extruders can with advantage be operated in corotating or counterrotating mode.
For the inventive pressure sensitive adhesive it is of advantage if the block copolymers have an average molecular weight Mn (numerical average) of between 5 000 and 600 000 g/mol, in particular between 80 000 and 450 000 g/mol.
The fraction of the polymer blocks P(B) is preferably between 10 and 60% by weight, in particular between 50 and 40% by weight, of the overall block copolymer.
For its advantageous further development, up to 50% by weight, in particular from 20 to 40% by weight, of resins are added to the pressure sensitive adhesive of the invention. Examples of resins that can be used include terpene resins, terpene-phenolic resins, C5 and C9 hydrocarbon resins, pinene resins, indene resins, and rosins, alone or in combination with one another. In principle, though, it is possible to use any resins which are soluble in the corresponding polymer; in particular, mention may be made of all aliphatic, aromatic and alkylaromatic hydrocarbon resins, hydrocarbon resins based on pure monomers, hydrogenated hydrocarbon resins, functional hydrocarbon resins, and natural resins.
For a very preferred development, highly apolar hydrogenated or partly hydrogenated hydrocarbon resins are admixed to the block copolymer, and take up residence preferentially in the P(A) domains. As a commercially available resin, mention may be made here of Regalite R 91(trademark) from Hercules.
Independently of the admixture of resin, it may further be advantageous to add further additives, particularly compounding agents, aging inhibitors, light stabilizers, ozone protectants, fatty acids, plasticizers, expandants, accelerators and/or fillers (for example, carbon black, TiO2, solid or hollow beads of glass or other materials, nucleators).
Additionally and optionally, crosslinkers are added to the block copolymer. Suitable crosslinkers include, for example, metal chelates, polyfunctional isocyanates, polyfunctional amines or polyfunctional alcohols. It is also possible to use polyfunctional acrylates with advantage as crosslinkers for actinic radiation.
Alternatively, UV photoinitiators are added to the block copolymers. Useful photoinitiators whose use is very favorable include benzoin ethers, such as benzoin methyl ether and benzoin isopropyl ether, for example, substituted acetophenones, such as 2,2-diethoxyacetophenone (available as Irgacure 651 from Ciba Geigy), 2,2-dimethoxy-2-phenyl-1-phenylethanone, dimethoxyhydroxyacetophenone, for example, substituted alpha-ketols, such as 2-methoxy-2-hydroxypropiophenone, for example, aromatic sulfonyl chlorides, such as 2-naphthylsulfonyl chloride, for example, and photoactive oximes, such as 1-phenyl-1,2-propanedione 2-(O-ethoxycarbonyl)oxime, for example. Additionally, polymers used for the pressure sensitive adhesives of the invention can be UV-crosslinked directly, without adding UV photoinitiators, if photoinitiators have been copolymerized in the polymer. In general, coinitiators or accelerators assist the crosslinking. It is possible to use all UV-crosslinking-accelerating substances which are known to the skilled worker.
A further development which makes the process for preparing the pressure sensitive adhesives of the invention particularly advantageous for the preparation of, for example, adhesive tapes is distinguished by the further processing of the blended pressure sensitive adhesive from solution or the melt and by its application in particular to a backing.
Usable backing materials for adhesive tapes, for example, include those materials which are customary and familiar to the skilled worker, such as films (polyester, PET, PE, PP, BOPP, PVC), webs, foams, wovens and scrimmed films, and also release paper (glassine, HDPE, LDPE). This list is not intended to be conclusive.
In the case of crosslinking of the pressure sensitive adhesive, said adhesive is treated preferably with actinic radiation. Crosslinking of the hotmelt pressure sensitive adhesives of the invention takes place by brief UV exposure in the range from 200 to 400 nm using commercial high or medium pressure mercury lamps with an output of, for example, from 80 to 200 W/cm, or by means of thermal crosslinking within a temperature range between 70 and 140xc2x0 C., or by means of ionizing radiation, such as by electron beam curing, for example. For UV crosslinking it may be appropriate to adapt the lamp output to the belt speed or, in the case of slow travel, to partially shade off the belt in order to reduce its heat exposure. The exposure time depends on the model and output of the radiation sources in question.
The invention further provides for the use of a pressure sensitive adhesive as described above for an adhesive tape provided on one or both sides with the pressure sensitive adhesive, particularly for an adhesive tape for bonds to apolar surfaces, said adhesive tape being produced preferably by applying the pressure sensitive adhesive from the melt to a backing.
Finally, the invention also embraces a process for preparing a pressure sensitive adhesive as set out hereinabove, in which the block copolymer is prepared by a thioester- or thiocarbonate-controlled radical polymerization.